Does anyone have an interesting way to measure negative voltages with an A-to-D in the 0 to 4v range?

The normal way to do this is to use an op-amp in inverting mode. You can put then use a voltage divider on the input of the inverter to scale the signal. Note that the inverting op-amp may have an input impedance low enough that it needs to be taken into account when designing the divider. The common mode rejection of the op-amp will prevent variations in supply voltages from affecting accuracy.